WO1995005722A1 - Radio distribution port systems - Google Patents

Abstract

A radio communication system, more especially for mobile handsets in a microcellular system, wherein a single TDMA transceiver (10) connects to a plurality of separate RDPs (14, 16, 18), one at a time, in different timeslots, so that secondary stations (20, 22) within the coverage area of an RDP can communicate with the shared transceiver during the timeslot allotted to that secondary station.

Description

RADIO DISTRIBUTION PORT SYSTEMS

The present invention relates generally to radio communication systems and in particular to a digital time division multiple access (TDMA) radio communication system which employs multiple cells to provide an adequately large coverage area.

Several known digital radio communication systems, e.g. GSM, DECT, use time division multiple access (TDMA) . A TDMA radio channel is organised into a series of time slots and a central base station communicates with a designated secondary station during the time slot allocated to that secondary station.

The area served by each radio base station is known as a cell. The maximum size of a cell is limited by system parameters such as maximum transmitter power and by the radio propagation conditions in the cell. In order to provide coverage over areas larger than a single cell, multiple cells are used. The cells are connected to a central switching station. Each cell typically comprises an antenna, transceiver equipment, control equipment and equipment for communicating with the central switching station.

Some known TDMA systems use high gain antennae to increase link gain and to minimise interference from and to adjacent cells. Because such high gain antennae are physically- large and cumbersome, these systems use a single, εteerable (or scanning) very high gain antenna. During each time slot this is pointed at the appropriate secondar- station. An object of the present invention is to reduce the cost of providing coverage in multi-cell TDMA radio communication systems by allowing a single transceiver to be shared between a plurality of cells.

In the following text, the term radio distribution port (RDP) is used to indicate the device which emits and receives radio signals. In some embodiments the RDP will comprise an entire transceiver and antennae assembly. In others it will comprise an antenna only. In others it will ,comprise an antenna and frequency changing means.

According to the invention there is provided cable means for connecting a single TDMA transceiver to a plurality of geographically separate RDPs wherein, one at a time, the RDPs are connected to the common transceiver during different timeslots so as to allow secondary stations within the coverage area of an RDP to communicate with the shared transceiver via the RDP antenna during the timeslot allotted to that secondary station.

More particularly the system in accordance with the present invention comprises a base station transceiver means, signal feeder cable means, a synchronised feeder switching means, radio distribution port means, and secondary stations.

The RDPs are located remotely from the transceiver and are connected to it using the feeder cable means. Each operative secondary station (making a call) is located within the coverage area of one of the RDPs and communicates with the TDMA transceiver during the allotted timeslot of that secondary station. Timeslots are allotted to secondary stations at the beginning of a call and at the end of the call are released for use by other secondary stations. The synchronised feeder and switching means connects the appropriate RDP to the transceiver during said timeslot. In the following timeslot a different RDP may be connected to the transceiver, allowing a secondary station in the coverage area of this different RDP to communicate with the transceiver during the timeslot allotted to this secondary station.

Each RDP is allotted a unique identity and the transceiver transmits this identity in the signalling channel of calls in progress. When there are no calls in progress via an RDP the transceiver transmits a "dummy bearer" or "beacon" signal containing the identity of the RDP. Thus from its externally observable behaviour, the arrangement behaves as if it were a plurality of different base-stations. This broadcast signal also supplies secondary stations with information on which timeslots are available for use on the respective RDP.

Radiocommunication systems often use antenna diversity at the base-station. In this invention where the antenna is physically remote from the base antenna diversity is provided using a pair of antennae at the RDP and the control signal to select between these two is multiplexed onto the feeder cable or carried on an auxiliary cable.

When there is a call in progress via one RDP and the user travels out of this area and into the area served by another RDP, the system may handover the call to the new RDP. Pre-handover measurements may be made either by the secondary station or by the base station transceiver. In order to reduce the signal attenuation along the feeder, the signal may be amplified, converted to another radio frequency, or converted to an optical frequency in the case of a fibre optic feeder.

The present invention is now further explained and described by way of example with reference to the accompanying drawings, wherein:-

Figure la shows a single base station transceiver time division multiplexed between a plurality of RDPs in a star configuration;

Figure lb shows a single base station transceiver time division multiplexed between a plurality of

RDPs connected in a bus configuration;

Figure lc shows a hybrid bus/star configuration;

Figure Id shows an evolution of Figure lc where additional transceivers have been added in order to increase capacity;

Figure 2 shows the TDMA duplex frame and time slot structure of the systems and also shows the composition of an individual timeslot;

Figure 3 shows possible methods of distributing control signals to the RDPs;

Figure 4 shows transmission amplifiers used in the feeder -cables;

Figure 5 shows an arrangement for remote control diversity; and

Figure 6 shows frequency changing means at both ends of the feeder cable.

Referring to the embodiments shown in Figure la to Figure Id, the system comprises a TDMA base station transceiver 10 connected via feeder cable means 12 to a plurality of radio distribution ports (RDPs) 14, 16, 18. The RDPs are geographically distributed and the coverage area of each may be arranged to be contiguous or partially overlapping with coverage area of adjoining RDPs.

The transceiver 10 can communicate via the RDPs 14, 16, 18 with specific secondary stations (e.g. handsets) 20, 22 respectively during specific timeslots allotted to the secondary stations. Figure 2 shows the structure of a frame and of a timeslot.

Thus, during the timeslot designated for communication with a particular handset, a feeder means and a switching means connect the appropriate RDP to the transceiver via the feeder cable means.

Radiocommunication systems often use antenna diversity at the base-station. In this invention, where the antenna is physically remote from the base-station antenna diversity is provided using a pair of antennae at the RDP and the control signal to select between these two is carried along the feeder cable or on an auxiliary cable. An arrangement for remote control diversity is shown in Figure 5, wherein the base-station diversity switch is referenced 40, the remote diversity switch 42, the feeder cable 43 and the control signal cable means 45. When there is a call in progress via one RDP and the user travels out of this area and into the area served by another RDP, the system may handover the call to the new RDP. Pre-handover measurements may be made either by the secondary station or by the base-station trans¬ ceiver. The method by which the secondary station effects measurement is well known. The base-station transceiver may make measurements of the signal strength of prospective "new" base-stations during the synchronisation header period of the timeslot, since it is not necessary to receive the entire synch signal on every timeslot.

The feeders may be connected in either a star configuration (Figure la) or a bus configuration (Figure lb) or in a suitable combination of the two. When a star configura¬ tion is employed the slot/RDP selection switch 39 may be collocated with the transceiver. Using this configura¬ tion it is also possible to use coaxial cable for some branches and to use optical fibre for other branches.

When a bus configuration is used, the switching function 41 is distributed along the length of the feeder. In this case it is necessary to supply a slot/RDP selection signal along the feeder.

In both cases a frame synchronisation signal is provided to control the transmit/receive cycle of the transmission amplifiers. The necessary synchronisation may be achieved in a variety of known ways. An example is given later, wherein a timeslot or part timeslot selection signal is provided to each RDP.

In some embodiments, as indicated in Figures la to Id, each RDP may comprise an antenna only. However with long feeder lengths this may prove to be impractical as the signal is greatly attenuated along the feeder between the transceiver and the antenna. Thus in other embodiments at least some of the RDPs may comprise amplifier means 24 and/or frequency changing means 26, as indicated in Figure 6. By changing the frequency at which the signal is transmitted along the feeder cable either to a much lower frequency, e.g. 100 MHz, or to an optical frequency in the case of an optical fibre feeder, the attenuation is reduced.

Other amplifiers operating at the feeder transmission frequency may be employed along the length of the feeder cables. Figure 4a shows how separate amplifiers 30, 32 may be used for transmit and receive or a single amplifier 34 may be switched to act in both directions. Figure 4b shows how the amplifiers 36, 38 may be inserted at either end of a length of cable so as to maximise noise performance. The feeder may comprise standard fixed lengths, e.g. 3m, 10m, 30m, 100m, which are factory adjusted so that the amplifier gain exactly matches the cable loss.

There are two basic methods of carrying the control signals from the transceiver to the RDPs. They may either be multiplexed on the feeder cable itself or may be carried on an auxiliary cable or cables.

In the embodiment shown in Figure 3a, the frame synchronisation signal is produced by a clock circuit 50 and is multiplexed onto the feeder cable in the form of a sine wave. This is then squared and sliced by a clock recovery circuit at the transmission amplifier. This clock recovery circuit has a long time constant to allow other serial data to be multiplexed on without affecting the clock jitter. The RDP/slot selection signal is produced by the circuit means 52 and is modulated serially to each RDP where it is decoded by decoder 46.

In the embodiment shown in Figure 3b, the frame synchronisation is carried on an auxiliary cable and is receovered by a clock recovery circuit 48. The timeslot/ RDP selection signal is provided on an auxiliary parallel bus, i.e. an n-core auxiliary cable which directly addresses up to 2 RDPs. Another wire pair carries the antenna diversity selection signal.

Additional capacity may be added by adding additional transceivers (Figure Id). This may be accomplished by connecting one of the existing feeders, or a new feeder, to an additional transceiver.

Claims

Claims

1. A radio communication system comprising cable means for connecting a single TDMA transceiver to a plurality of geographically separate RDPs wherein, one at a time, the RDPs are connected to the common transceiver during different timeslots so as to allow secondary stations within the coverage area of an RDP to communicate with the shared transceiver via the RDP antenna during the timeslot allotted to that secondary station.

2. A system according to claim 1, comprising a base- station transceiver means, signal feeder cable means, RDPs and secondary stations, wherein the transceiver means transmits a unique and different identity for each different RDP.

3. A system according to claim 2, wherein signals on the feeder cable means are transmitted at the same radio frequency as that used between the secondary station and the RDP.

4. A system according to any of claims 1 to 3, having antenna diversity at the RDP which is remotely controlled.

5. A system according to claim 2 or claim 3 when appendant to claim 2, having amplifier means in the feeder cable means being either a bi-directional switched Tx/Rx transmission amplifier or unidirectional transmission amplifiers, wherein signals on the feeder cable means are transmitted at an optical frequency modulated by the radio frequency signal.

6. A system according to any of claims 1 to 5, wherein the feeder cables are either arranged in a bus or star configuration, or in a hybrid configuration of the two.

7. A system according to any of claims 1 to 6, having base-station or secondary station signal quality measurement means enabling transfer of a secondary station in use from one RDP to another.

8. A system according to claim 7, wherein the base- station samples the signal qualities of RDPs neighbouring the one in use during the synchronisation period in order to make a handover decision.

9. A system according to any of claims 1 to 8, wherein additional transceivers can be added to an existing or new feeder cable.

10. A system according to any of claims 1 to 9, wherein control signals are multiplexed onto the feeder cable means or carried on an auxiliary cable.